1. Field of the Invention
This invention relates to a DA (digital-to-analog) converter, and mainly to techniques which are effective when utilized for a DA converter of current addition type permitted to operate with a low supply voltage.
2. Description of the Prior Art
Examples of a DA converter of current addition type are stated in the official gazettes of Japanese Patent Applications Laid-open No. 191290/1993 and No. 120834/1994. In the current addition type DA converter, a transistor (MOSFET: metal oxide semiconductor field-effect transistor) for forming a constant current is operated in its saturation region so as to construct a constant-current source. Therefore, a required supply voltage is a comparatively large voltage which is obtained by adding a lower-limit voltage for operating the transistor in a non-saturated condition, to a maximum output voltage.
Moreover, the maximum output voltage (full-scale voltage) becomes a voltage produced when all the constant currents are added up and are caused to flow through a load resistor. It is influenced by discrepancy in the individual constant currents, and is difficult to be set at a desired voltage. In particular, when the DA converter is operated with a low supply voltage, currents in the individual constant-current source transistors are liable to decrease under the influence of an output voltage. Therefore, it becomes increasingly difficult to set the maximum output value at the desired voltage.
Accordingly, this invention has for its object to provide a DA converter which is permitted to operate with a low voltage. Another object of this invention is to provide a DA converter which facilitates the setting of its maximum output voltage. Still another object of this invention is to provide a DA converter which is permitted to operate with a low voltage, and which facilitates the setting of its maximum output voltage. The above and other objects and novel features of this invention will become apparent from the description of this specification when read in connection with the accompanying drawings.
A typical aspect of performance of this invention is briefly summarized as follows: In a DA converter wherein an analog output voltage is formed in such a way that constant currents formed by a plurality of constant-current source transistors are selectively fed to a load resistor by controlling switching means to turn ON and OFF in correspondence with input signals; each of the constant-current source transistors is operated in an operating range from a saturation region to a non-saturation region as the output voltage enlarges in its absolute value, and those of the plurality of constant-current source transistors whose currents based on the operations in the non-saturation regions are selected by the switching means have their sizes enlarged so as to compensate for current decrements ascribable to such operations in the non-saturation regions.
Another typical aspect of performance of this invention is briefly summarized as follows: A DA converter comprises a current mirror circuit which produces an analog output voltage in such a way that constant currents formed by a plurality of constant-current source transistors are selectively fed to a load resistor by controlling switching means to turn ON and OFF in correspondence with input signals, and a reference voltage generation unit which forms a constant voltage for forming the constant currents; the plurality of constant-current source transistors include ones which have weights of currents corresponding to binary digital signals of lower bit side among the input signals, and ones which have the same weight of currents in correspondence with digital signals obtained in such a way that binary digital signals of higher bit side among the input signals are decoded in the decimal notation; and a voltage formed in such a way that a current formed by a transistor, which is fabricated with the same size as that of each of the constant-current source transistors having the same current weight correspondent to the higher bit side and whose emitter (or source) and base (or gate) are respectively connected in common with those of the constant-current source transistors, is caused to flow through a resistance element endowed with a resistance being that multiple of a resistance of the load resistor which corresponds to the number of the constant-current source transistors correspondent to the higher bit side, and a reference voltage corresponding to a maximum output voltage to-be-set, are fed to the input ends of a differential amplifier circuit, whereby the currents to flow through the constant-current source transistors are controlled so as to equalize both the voltage across the resistance element and the reference voltage.